1. Field of the Invention
The present invention relates to a cellulose acylate film and its production method, and to a polarizer and a liquid crystal display device comprising the cellulose acylate film. In particular, the invention relates to a cellulose acylate film favorable for use as an optical film such as a polarizer protective film, an optical compensatory film, etc.
2. Description of the Related Art
With the recent tendency toward advancing TV use of liquid crystal display devices, the panel size of the devices is enlarged and high-definition and low-price liquid crystal display devices are much desired. In particular, VA-mode liquid crystal display devices have a relatively high contrast and enjoy a relatively high production yield, and are therefore most popular liquid crystal display devices for TV use.
However, VA-mode liquid crystal display devices have a problem in that, at the time of black state, the devices could provide black that is good in some degree in the normal direction to the display panel, but when the black state panel is seen in viewing angle directions (oblique directions), there occurs light leakage to disable background black display whereby the viewing angle is narrowed. Accordingly, a retardation film is desired capable of expressing a retardation level in such a degree that enables viewing angle compensation.
Recently, further, for preventing the neutral tone on a liquid crystal display panel from being yellowed, a multigap (MG) cell has become used in which the thickness of the liquid crystal layer, or that is, the cell gap is changed for every color. However, the multigap cell is problematic in that, as compared with that on a conventional liquid crystal display panel, the color shift at the time of black state in viewing angle directions increases, and therefore, it has become much desired to further improve the multigap cell in point of preventing the color shift at the time of black state in viewing angle directions on a liquid crystal display panel.
On the other hand, the demand for use of liquid crystal display devices in various environments has become increased, and in particular, the demand for favorable use thereof in high temperature and high humidity environments, for example, for outdoor use thereof has increased. In case where a retardation film of which the dimensional change in use in high temperature and high humidity environments is large is incorporated in a liquid crystal display device, there may occur corner unevenness on the liquid crystal display panel, and therefore, it is desired to improve the durability of the film in high temperature and high humidity environments.
Further improvement of the display performance of liquid crystal display devices and further reduction in the production cost thereof are still desired, and it is desired to provide an inexpensive retardation film capable of fully attaining viewing angle compensation even though it is thin, and capable of enabling further contrast increase.
Regarding the requirements, it is known that use of a retardation film having reversed wavelength dispersion characteristics of retardation, or that is, a retardation film having optical properties of such that its in-plane retardation Re increases on a longer wavelength side is effective for preventing color shift at the time of black state in viewing angle directions on a liquid crystal display panel (see JP-A 2009-1696).
Films having reversed wavelength dispersion characteristics of retardation that have heretofore been investigated are produced by adding an additive having a negative intrinsic birefringence to a resin film. However, the additive having a negative intrinsic birefringence is expensive and has some problems in that, when such an additive having a negative intrinsic birefringence is added to a resin film, then the thickness-direction retardation Rth of the film lowers and therefore, in order to make the film express a desired retardation level, the thickness of the film must be increased or the amount of the retardation enhancer to be added to the film must be increased, and as a result, from the viewpoint of the material cost, the additive is unsatisfactory.
As opposed to this, JP-A 2009-1696 discloses a technique of adding an acrylic polymer having a negative intrinsic birefringence and further a sugar ester compound to a phthalyl/acetyl-modified heterogeneous cellulose-mixed cellulose acylate, thereby improving the light leakage resistance, the color shift resistance, the front contrast, the retardation and the wavelength dispersion characteristics of retardation of the resulting film. This patent reference suggests the possibility of stretching the film, but says nothing about high temperature and high humidity treatments of the film after stretching.
On the other hand, for improving the dimensional stability of films in high temperature and high humidity environments, JP-A 2002-179819 discloses a method of once drying a formed cellulose ester film followed by humidifying the film to thereby increase the water content of the film. The patent reference says that it is preferable to stretch the dry film in a film width direction (direction perpendicular to the film conveying direction) by from 1 to 20% or so at 80 to 150° C. or so, disclosing examples where a film is, after stretched, humidified at 70° C. and at a relative humidity of 80% or in a water bath at 40° C. However, the patent reference says nothing about use of a sugar ester compound.